Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that can modulate cortical excitability of targeted brain regions. Various studies have investigated tDCS use in stroke patients with motor impairments (cumulatively about 200 cases). Although these studies are mostly "proof of concept" with small sample size, they do suggest that tDCS may improve motor function. However these two questions have not been addressed systematically: